EUP is set to not Editable for Manager field, Multi User Requests Manager field show as Editable.
Solution
In Single User Request, you can add any User which is valid GRC user. It will create a Request & you have full options to make any User a Manager.
Self & Others Requests works with EUP, whereas EUP is not supported for Multi-user Requests.
There is no way where Manager field can be grayed out for Multi-User. Even the 5021 Parameter would just validate, whether it is correct GRC System User or not.
So, Manager User field will be editable in Multi User Requests.